python 3.12 compatibility

[soxofaan]

just tried to enable the github actions test workflow for python 3.12 but it failed with

 tests/test_codecs.py:3: in <module>
    from dahuffman import load_json, load_json_compact
/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/site-packages/dahuffman/__init__.py:2: in <module>
    from dahuffman.codecs import (
/opt/hostedtoolcache/Python/3.12.0/x64/lib/python3.12/site-packages/dahuffman/codecs/__init__.py:11: in <module>
    import pkg_resources
E   ModuleNotFoundError: No module named 'pkg_resources'